Superweed Palmer Amaranth is on the Move: Here’s What You Need to Know
Palmer amaranth (Amaranthus palmeri) is an aggressive, annual, broadleaf weed native to the desert regions of the Southwestern U.S. It has slowly invaded this area of the country where it is considered the most severe weed of cotton and soybean. Recently, increased movement of contaminated animal feed, manure, harvest equipment, and conservation seed plantings has caused the weed to spread quickly in the Midwest.
To date, this weed has been detected in 28 states, most recently in Minnesota. From 2016-2017, Palmer amaranth has been detected in prairie conservation plantings across four Minnesota counties with eradication efforts underway. Alarmingly, bioclimatic models indicate the potential for further spread across the state. There is a high risk for establishment in large portions of North Dakota where Palmer amaranth has not yet been detected.
What Makes Palmer Amaranth so Problematic?
This weed has the perfect combination of weedy and invasive traits. Palmer amaranth is known for its rapid growth rate of about 2-3 inches per day and often reaches heights of 6 to 8 feet. It also has an extended germination period with plants emerging from March into October in their native range, and from early May into mid-September in Midwestern states.
In turn, Palmer amaranth forces producers to manage the weed throughout the year, unlike other summer annual weeds that are typically managed only through early summer. To make matters worse, Palmer amaranth is a prolific seed producer. The weed can produce 100,000 seeds when competing with crops and upwards of a half million seeds in noncompetitive scenarios.
Palmer amaranth is very difficult and expensive to control due in part to its ability to develop resistance to multiple herbicide families.
This weed has separate male and female plants. Pollen from male plants spread via the wind to female plants (outcrossing), resulting in more genetically diverse populations. This also enables the rapid development and transfer of adaptive traits (including herbicide resistance) across agricultural lands. Since their initial detection in Georgia in 2004, glyphosate-resistant populations of this weed have become widespread throughout the Southeastern U.S. and are now found in Midwestern states.
Which Crops Are at Risk?
While Palmer amaranth is notorious among cotton, corn, and soybean growers, numerous vegetable crops can be outcompeted by this adaptive weed. Palmer amaranth infestations cause substantial yield losses in sweet corn, bell peppers, tomatoes, and sweet potatoes. Unfortunately, the impact of this weed on many horticultural crops is poorly understood, and additional research is greatly needed.
Growers employing no till or reduced tillage management practices are especially at risk from this pigweed. In those cases, seeds are allowed to stay in their ideal emergence zone: the top inch of the soil surface. These tiny seeds can survive in the topsoil for one to three years where they are often picked up and spread across the landscape by farm equipment.
Identification Characteristics to Note
Palmer amaranth looks similar to other widespread pigweeds including common waterhemp, redroot, and smooth pigweeds. To correctly identify the pigweed species in your field, note the following characteristics.
The presence of hair. Redroot and smooth pigweeds have hair on their stems and leaves. Palmer amaranth and waterhemp do not have hair on any surface.
Leaf characteristics. Green leaves are typically smooth and arranged in an alternative pattern that grows symmetrically around a course green to reddish stem. Common waterhemp leaves are generally long, linear, and lance-shaped. In comparison, Palmer amaranth leaves are wider and ovate to diamond-shaped. Some leaves have a whitish, v-shaped mark or a sharp spine at the leaf tip.
Another way to tell them apart is to examine the length of stalk connecting the leaf to the stem or petiole. Palmer amaranth’s petiole is longer than the length of the leaf. For common waterhemp, the petiole will only be about half the length of the leaf.
Seed head structures. Palmer amaranth female plants have a long, terminal seed head spike that can reach up to 3 feet long. These seed heads are sharp and prickly to the touch. The seed heads of common waterhemp, redroot, and smooth pigweeds tend not be as prickly.
For additional information, Kansas State University Extension published an excellent pigweed identification guide complete with photo illustrations and keys to distinguish mature plants of nine different pigweeds, which you can download for free.
Manage for Long-Term Control
Growers need to be proactive when it comes to Palmer amaranth scouting, identification, and management. Actively search for it in your fields, boarders, ditches, and along roadsides. Eliminating the weed before it can produce seeds is best but not always feasible.
Integrated weed management programs will be the key to sustainable long-term control of Palmer amaranth. Consider combining cultural and chemical methods. Recommended cultural practices include deep tillage, mulching, crop rotations, cover crops, and hand weeding. Utilizing a combination of chemical and non-chemical tactics will help reduce selection pressure for herbicide resistance.
Consult with your local university/county Extension specialist or crop advisor on developing an effective chemical management plan as herbicidal resistance varies greatly across this weed’s wide geographic range. Many populations are already resistant to glyphosate and acetolactate synthase, also known as ALS herbicides. To prevent further spread, work in areas where the weed is absent first and in infested areas last. After working in infested areas, be sure to clean farm equipment (especially combines).